1. Field of the Invention
The present invention relates to a sheet feeder and an image forming apparatus, and more particularly, to a mechanism for feeding sheets one sheet by one sheet.
2. Description of the Related Art
A conventional image forming apparatus such as a printer or a copier includes a sheet feeder for feeding sheets to an image forming portion. Some sheet feeders include a paper feed roller for feeding sheets set in a sheet tray, a sheet separator for separating for separating the fed sheets one sheet by one sheet, and a convey roller which is provided downstream of the sheet separator and which conveys the separated sheet. The sheet separator brings the separation pad into contact with the paper feed roller under pressure, and the sheets are separated from each other between the paper feed roller and the separation pad. In the separation pad type sheet separator, the separated sheet is conveyed by the convey roller and the sheet is sandwiched between the paper feed roller and the separation pad. Therefore, this sheet separator has a problem that a back tension (load) when the sheet is conveyed is high. Hence, there is a technique in which the separation pad is separated from the paper feed roller after the sheet is separated, the sheet is applied a force toward the separation pad by a rotatable roller, thereby reducing the back tension when a sheet is fed. This technique is disclosed in Japanese Patent Application Laid-open No. 2001-7221.
A conventional sheet feeder which applies a force of a rotatable roller toward the separation pad will be described with reference to FIGS. 11.
According to a sheet feeder shown in FIG. 11A, in an initial state of the sheet feeding operation, a middle plate 102 which can be rocked by a release cam (not shown) is pushed down to a predetermined standby position. With this, a sheet P on the middle plate 102 and a paper feed roller 103 are isolated from each other. The separation pad holder 104 is pushed down through a separation pad holder pressing portion 105 of the middle plate 102 and thus, the separation pad 106 is isolated from the paper feed roller 103. The roller 107 is applied a force toward the separation pad 106 under predetermined pressure. A return lever 108 which returns a sheet lying from the middle plate 102 to the separation pad 106 completes its returning operation, and is located on a state where the return lever 108 is retreated from a convey path of the sheet P.
If the sheet feeding operation is started, as shown in FIG. 11B, the middle plate 102 starts moving upward. At that time, the position restriction where the separation pad holder pressing portion 105 of the middle plate 102 acts is released and with this, the separation pad holder 104 starts moving upward. The paper feed roller 103 and the separation pad 106 abut against each other before the paper feed roller 103 and the sheet P on the middle plate 102 abut against each other by the setting of the action stroke. Therefore, even if the sheet P is brought into an unstable state by the movement of the middle plate 102, since the separation pad 106 and the paper feed roller 103 abut against each other first, the sheet P does not enter forward of the separation pad 106.
Next, the paper feed roller 103 and the middle plate 102 are pressed through the sheet P as shown in FIG. 11C. Then, as shown in FIG. 11D, the sheet P is picked up as the paper feed roller 103 rotates, and the paper feeding operation is carried out. If the uppermost fed sheet P passes through the sheet tip end detecting sensor (not shown) and is sandwiched between a convey roller (not shown) and a pinch roller (not shown), the separation pad 106 is separated from the paper feed roller 103. At that time, the separation pad 106 and the paper feed roller 103 are separated from each other after the middle plate 102 and the paper feed roller 103 are separated from each other as shown in FIG. 11E. With this, even if the set sheets become unstable by the movement of the middle plate 102, since the separation pad 106 and the paper feed roller 103 abut against each other, the sheet does not come forward of the separation pad 106. The sheet P which is conveyed and on which an image is formed is applied a force to the separation pad 106 by the roller 107, subsequent sheet is prevented from falling.
If image formation onto the sheet P is carried out and the discharging operation is completed, the return lever acting cam 110 rotates to act on the return lever 10 as shown in FIG. 11F, and the sheet lying on the separation pad 106 is returned to a loading position. If the return lever acting cam 110 further rotates and does not act on the return lever 108, the return lever 108 returns to its initial position shown in FIG. 11A by its own weight.
When the above-described series of sheet feeding operation is completed and the next sheet P is to be fed, the above-described operation is repeated.
The sheet feeder described in the description of the related art includes the rockable middle plate, and the loaded sheets are fed by moving the middle plate. The paper feed roller and the separation pad are separated from each other during the paper feeding operation in association with movement of the middle plate. Therefore, back tension generated by them is reduced.
In the mechanism for reducing the back tension as described above, however, there is a problem that the application of the mechanism is limited to a sheet feeder using the middle plate. Further, in the mechanism for reducing the back tension, a relatively large member, i.e., the middle plate must move. Thus, it is difficult to simplify and reduce the size of the mechanism itself and thus, the entire sheet feeder.
If the sheet feeder is reduced in size, a guide space through which a sheet is conveyed becomes narrow. As a result, the conveying direction of a sheet is changed many times and the curve of each charge becomes acute. Thus, when the feeding direction is abruptly changed immediately after the sheet passes through the paper feed roller, there is a problem that a rear end of the sheet jumps and an impact noise is generated.